The other day I worked out with my sister-in-law, who is a personal trainer and very, very fit. I was nervous that she'd intimidate me, but instead, I was totally motivated by her drive and skills. Ordinarily I may not push myself because I have no point of reference when I am working out alone, but working out with a super-fit friend motivated me to workout just a little harder than I normally would.
I know that it could seem intimidating to get a fit buddy as a workout partner, but as long as your fit workout partner is a nice person, working out alongside her will challenge you to push your own limits (run a little faster, lift a little heavier weight, etc.).
You don't have to go as far as enlisting a personal trainer, but what about your friend who goes to the gym every other day? Or what about your brother who is a marathon runner? Maybe your husband could even be a fit buddy for you.
Then one day you could be the fit buddy that someone else needs to help them along in their own workout endeavors.